1. Field of the Invention
The present invention relates to a metal corrosion inhibitor for preventing corrosion of metal in a steam generating unit such as a boiler, steam generator or the like, or in a petroleum refining or petrochemical process unit, and to a method for preventing corrosion of metal in a steam generating unit or petroleum refining or petrochemical process unit, and relates in particular to a metal corrosion inhibitor and method for preventing corrosion of metal which can prevent corrosion even when small quantities of the metal corrosion inhibitor are added. Moreover, the present invention relates to a hydrogen chloride formation inhibitor and method for inhibiting formation of hydrogen chloride which can prevent metal corrosion caused by hydrogen chloride gas in a crude oil atmospheric distillation unit.
2. Description of the Related Art
In order to prevent corrosion of metal in equipment such as various types of boilers, the waste heat boilers of petrochemical plants, gas turbine facilities and the like, and the steam generators of pressurized water reactors, it is necessary to control the pH appropriately at various points (the feed water system, the boiler, the vapor/condensed water system.) For example, in a boiler wherein softened water or raw water is feeded, carbonic acid is produced when carbonates and hydrogen carbonates contained in the feed water decompose in the boiler water. The resulting carbonic acid lowers the pH primarily in the vapor/condensed water system, and this causes corrosion.
Moreover, the steam generation unit of a process system may become contaminated with acid components including inorganic acid and organic acid derived from the process. Such acid components lower the pH at various points and cause corrosion.
Such corrosion can be prevented by neutralizing the acid components with a pH controller to obtain an appropriate pH.
Thus, ammonia and neutralizing amines are ordinarily used as pH controllers in feed water systems and vapor/condensed water systems. Examples of neutralizing amines include alkanolamines, cyclohexylamine, morpholine and the like.
Corrosion prevention in the boiler is normally achieved by controlling the pH using boiler compounds containing sodium hydroxide, sodium phosphate or the like, but when volatile treatments are also being processed in the boiler, ammonia and neutralizing amines are used as pH controllers. As above, possible neutralizing amines include alkanolamines, cyclohexylamine, morpholine and the like (see Japanese Patent Application Laid-open No. S61-34095, page 2, upper left column, lines 1-3).
In the fields of petroleum refining process and petrochemical process, hydrogen chloride, hydrogen sulfide, carbon dioxide and other acid components contained in the crude oil can dissolve in water (condensed water, free water, etc.) generated within or at the top of a distillation column or fractionating column or the like, and cause corrosion of the metal of the unit.
For example, in the atmospheric distillation process of petroleum refining, the crude oil is ordinarily first desalted in the desaltor, then heated to a prescribed temperature in the heating furnace and feeded to the atmospheric distillation column (main distillation column), where it is separated into naphtha fraction, kerosene, gas oil component, residue and the like. In this atmospheric distillation process, the magnesium chloride and calcium chloride in the crude oil produce hydrogen chloride gas by hydrolysis, as shown by the following reaction formula.CaCl2+2H2O (heating)-->Ca(OH)2+2HClMgCl2+2H2O (heating)-->Mg(OH)2+2HCl
The hydrogen chloride gas produced in this way dissolves in condensed water produced in the low temperature area of the top of the column, the overhead system or the like of the atmospheric distillation column to form hydrochloric acid, leading to unpreferable results including acid corrosion of the metal of the unit.
It is also necessary in the fields of petroleum refining and petrochemical process to control the pH in order to prevent corrosion of metal of the unit by acid components, and in general ammonia and neutralizing amines are used as pH controllers. Examples of neutralizing amines include alkanolamines, methoxypropylamine and the like (see Japanese Patent Application Laid-open No. H10-251639, page 2, left column, lines 12-24 and right column, lines 29-37).
In the atmospheric distillation process of petroleum refining, efforts are also made to prevent formation of the hydrogen chloride gas itself which is the cause of metal corrosion, with the following technologies being employed.
(1) A method of preventing formation of hydrogen chloride by adding sodium hydroxide to the crude oil at the desaltor outlet in order to convert the magnesium chloride and calcium chloride in the crude oil to stable (not hydrolyzing even when heated) sodium chloride.
(2) A method of neutralize the generated hydrogen chloride in the form of amine salts by adding alkanolamines to the crude oil at the desaltor outlet (Japanese Patent Application Laid-open No. H3-101801, claims).
However, the ammonia and neutralizing amines used as pH controllers as described above were only weakly neutralizing, and had to be added in large quantities. Such problems were particularly obvious when trying to prevent corrosion inside a boiler if volatile treatment was being processed.
When used as pH controllers, moreover, ammonia and neutralizing amines produce acid components and salts during the neutralization process, and since ammonia and neutralizing amines are only weakly neutralizing, the resulting neutralized salts (amine hydrochlorides, amine carbonates and the like) and neutralized salts of strong acids in particular (such as amine hydrochlorides) can dramatically lower the pH when they dissolve in water, thus causing secondary corrosion.
On the other hand, in the above method (1) as a technology for inhibiting formation of hydrogen chloride, if too much sodium hydroxide was added, the sodium deteriorated catalysts in the heavy oil direct desulfurization unit at the subsequent stage. So it was necessary to limit the amount of sodium hydroxide added even if the crude oil at the desaltor outlet had a high salt concentration. As the result, the inhibitory effect on formation of hydrogen chloride was not satisfactory. In fact, when a crude oil refining unit was being operated continuously for two years or more, catalyst in the heavy oil direct desulfurization unit which had deteriorated due mainly to sodium from the sodium chloride had to be changed once a year.
In the above method (2), if the operating temperature of the top of the atmospheric distillation column was lower than the melting point of the amine hydrochlorides produced by neutralization, the amine hydrochlorides accumulated within the atmospheric distillation column, causing problems such as corrosion, line blockage or the like within the unit. Moreover, when the amine hydrochlorides produced by neutralization were heated by the furnace of the atmospheric distillation column, they became partially hydrolyzed, again producing hydrogen chloride. So neutralization by adding amines again to the top of the column was needed.